Music is the art of arranging sound. It is one of the universal cultural aspects of all human societies. Music may be defined with styles that emphasize elements of organized sound, such as rhythm, volume and pitch. Rhythm may be specified with tempos, sometimes organized using meters, and often coordinating the variation and juxtaposition of pitch. Individual sounds possess timbres or texture, which heavily contribute to the music's overall character. It is sometimes more valuable to categorize music by era, scene, intent, or artistic inspiration. Individual periods of music are separated into pieces such as songs, tracks, symphonies, or so forth. Pieces may be composed and performed using a vast range of instruments, including the human voice. There are solely instrumental pieces, solely vocal pieces, pieces that combine singing and instruments, pieces with no sound, randomly generated pieces, and even pieces merely specifying an environment with no further sonic organization. In some musical contexts, a performance or composition may be to some extent improvised. Music is played in public and private areas, highlighted at events such as festivals, rock concerts. Music often plays a key role in social activities (such as dancing, karaoke singing, and attending concerts), religious rituals, rite of passage ceremonies, graduation and marriage celebrations, and cultural activities such as community choirs. It can be a hobby or a profession, like a teen playing cello in a youth orchestra or a local funk band hired for parties. The music industry includes songwriters, performers (including orchestra, jazz band and rock band musicians, singers and conductors), sound engineers, producers, tour organizers, distributors of instruments, accessories, and sheet music. Compositions, performances, and recordings are assessed and evaluated by music critics, music journalists, and music scholars, as well as amateurs.